1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copy machine, a printer, a facsimile, and a plotter, and more particularly to the image forming apparatus having a rotatable transfer member capable of stably transferring an image on an image carrier by eliminating a variation in a transfer characteristic.
2. Discussion of the Related Art
Many electrophotographic devices, such as a color copy machine and a color printer, have a function of printing a color image.
Color electrophotographic devices can be generally classified as a one drum type and a tandem type. The one drum type includes a plurality of color developing devices disposed around one photoconductor. These developing devices deposit toner on the photoconductor to form a composite toner image and then transfer the image to develop a color image on a sheet. The tandem type includes a developing device for each of a plurality of photoconductors arranged horizontally to form a single color toner image on each photoconductor. The single color toner images are sequentially transferred to a sheet to develop a composite color image.
When comparing the one drum type and the tandem type, the following characteristics may be discerned. An advantage of the one drum type is that the one photoconductor makes the device relatively compact and inexpensive. However, the one photoconductor is required to form an image two or more times (generally 4 times) to develop a full color image. This process requires a considerable amount of time.
On the other hand, an advantage of the tandem type is that the plurality of photoconductors forms the full color image more quickly. However, the plurality of photoconductors makes the device relatively larger and more expensive.
A tandem type generally performs a monochrome printing, at a same speed as full color image formation.
The tandem type electrophotographic device includes a direct transfer system and an indirect transfer system. In the direct transfer system, four transfer units for the colors of Y, C, M, and Bk sequentially transfer images on respective photoconductors arranged horizontally to a recording sheet which is conveyed by a sheet transfer belt in a form of an endless belt. In the indirect transfer system, primary transfer units sequentially transfer images on respective photoconductors arranged horizontally to an intermediate transfer member in a form of endless belt. Subsequently, a secondary transfer unit simultaneously transfers the image to a sheet. The secondary transfer unit employs a transfer belt system. The secondary transfer unit may employ a roller system.
One exemplary image forming apparatus is described in Japanese Laid-Open Patent Application Publication No. 11-24507.
In the above mentioned systems, overlaying a plurality of color images of different colors (magenta, cyan, yellow and black toner images) on the transfer member without color shift is a significant challenge. Attempts have been made to rotate the sheet transfer belt and the intermediate transfer member at a constant rate.
One solution is to measure a surface speed with a linear encoder. The linear encoder performs a feedback control based on its output and uses the output to correct a timing for writing. This system provides alignment efficiently.
However, another problem arises. When a rotating member is an endless belt, an end of the belt is often rippled or distorted. Generally, tensioned endless belts tend to ripple at the end of the belts, causing loss in positional accuracy of liner encoders. An additional problem is that a scale for measuring a speed by linear encoders often causes static electricity, thereby affecting images and sheet conveyance.
The above-mentioned problems also apply to the one drum type color image forming apparatus. They become an impediment to enhancement of image quality.